An implantable medical device (IMD) is implanted in a patient to monitor, among other things, electrical activity of a heart and to deliver appropriate electrical therapy, as required. Implantable medical devices include pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (ICD), and the like. The electrical therapy produced by an IMD may include pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g., tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g., cardiac pacing) to return the heart to its normal sinus rhythm. These pulses are referred to as stimulus or stimulation pulses.
The stimulus pulses are delivered to chambers of the heart at a stimulus output. The stimulus output represents the strength or electrical potential of the stimulus pulses. If the stimulus output of the pulses is not sufficiently large, then the stimulus pulses may be ineffective. Stimulus pulses having an output that does not exceed a stimulation threshold of the heart may not cause polarization of one or more chambers of the heart or propagate. For example, a stimulus pulse applied to a ventricle at a stimulus output that is less than the stimulation threshold of the heart may not result in depolarization of the ventricle, or capture in the ventricle.
In order to ensure that stimulus pulses are delivered to the heart at electric potentials that exceed the stimulation threshold of the heart, the IMD may periodically adjust the stimulus output of the pulses. The IMD may invoke an autothreshold mode. In the autothreshold mode, the IMD performs a threshold search that adjusts the electric potential of stimulus pulses applied to the heart after the threshold search is completed. The threshold search adjusts the stimulus output of the pulses such that the pulses have an output that is at least as great as the stimulation threshold.
A threshold search in some known IMDs incrementally decreases the electric potential of pulses in consecutive cardiac cycles until a predetermined number of consecutive losses of capture are detected. For example, the stimulus output at which stimulus pulses are applied to a ventricle of the heart may be decreased by 0.25 or 0.3 Volts for each cardiac cycle until a loss of capture is detected. If a loss of capture is detected in a cardiac cycle, some known IMDs deliver a back-up stimulus pulse to the ventricle to ensure ventricular contraction during the cardiac cycle. Once the loss of capture is detected, the same stimulus output may be used for the next cardiac cycle to determine if another loss of capture occurs. If the loss of capture continues to occur for the predetermined number of consecutive cardiac cycles, then the losses of capture may indicate that the current stimulus output is below the stimulation threshold. In some known IMDs, the predetermined number of consecutive cardiac cycles is two. On the other hand, if the loss of capture does not occur in the following cardiac cycle, the stimulus output continues to be decreased in subsequent cardiac cycles until loss of capture occurs in the predetermined number of consecutive cardiac cycles.
Once the stimulus output is decreased such that loss of capture occurs for the predetermined number of cardiac cycles, the stimulus output is then incrementally increased. The stimulus output is increased for each cardiac cycle until capture is detected. For example, the stimulus output may be increased by 0.125 or 0.3 Volts for each cardiac cycle until capture is detected. Additional stimulus pulses are applied at the current stimulus output until capture occurs in a predetermined number of cardiac cycles. In some known IMDs, the predetermined number of cardiac cycles is two. The stimulation threshold is then set to be equal to this stimulus output. Additional stimulus pulses are applied at stimulus outputs that are at least as great as this newly established stimulation threshold. In some known IMDs, the additional stimulus pulses are supplied to the heart at levels that are at least 0.25 or 0.3 Volts above the stimulation threshold.
During the threshold search, stimulation pulses that do not result in capture in the ventricle within a predetermined time period after applying the stimulus pulse are followed with delivery of a back-up stimulus pulse. The back-up stimulus pulse is applied to the ventricle to ensure that the ventricle does contract or polarize. If the stimulus pulse is supplied at a sufficiently large stimulus output, the stimulus pulse will result in ventricular capture. But, delivery of the stimulus pulse and an intrinsic ventricular contraction may occur at approximately the same time and become conflated, or fused with one another.
The fusion of intrinsic and paced ventricular contractions may hamper the ability of the IMD to accurately perform the threshold search. Fusion may cause a captured ventricular contraction to appear as a non-captured ventricular contraction caused by a ventricular stimulus pulse. Alternatively, a non-captured ventricular contraction caused by a ventricular stimulus pulse may appear as a captured ventricular contraction.
Some known IMDs shorten the AV interval and/or PV interval during pacing associated with a threshold search in an effort to avoid fusion. The reduction of the AV and/or PV intervals is referred to as AV/PV shortening. AV/PV shortening may cause patient discomfort during threshold searches due to the relatively short AV and PV intervals. Shortening the AV and PV intervals frequently does not result in good hemodynamic response of the heart and may cause patients to be symptomatic.
A need exists for an improved threshold search that avoids fusion while maintains adequate hemodynamic response of the heart to improve patient comfort during the threshold search.